1. Field of the Invention
The present invention relates to liquid-ejecting heads, liquid-ejecting devices, and liquid-ejecting methods for ejecting an ejection medium contained in liquid cells from nozzles in a droplet form by driving energy units, and also relates to ejection media for the liquid-ejecting heads. Specifically, the present invention relates to techniques for providing liquid-ejecting devices with high ejection stability and a significantly wide operating temperature range.
2. Description of the Related Art
Inkjet printers are one of the known liquid-ejecting devices for ejecting an ejection medium, such as a liquid, contained in liquid cells from nozzles in a droplet form by driving energy units. A typical inkjet printer includes an inkjet head (a type of liquid-ejecting head) having nozzles arranged in line. The inkjet printer supplies ejection energy to ink by driving energy units to sequentially eject fine ink droplets from the nozzles to a recording medium, namely printing paper. The ink droplets land on the printing paper to form substantially circular dots arranged in two orthogonal directions, thus expressing images and characters.
Among the types of ink ejection of inkjet printers is thermal ejection, which is the ejection of ink by supplying heat energy thereto. A thermal inkjet printer includes an inkjet head having ink cells for containing an ink (ejection medium), heat-generating resistors (energy units) disposed inside the ink cells, and nozzles for ejecting the ink contained in the ink cells in a droplet form. This type of inkjet printer rapidly heats the ink by driving the heat-generating resistors to cause the film boiling of the ink on the heat-generating resistors and thereby produce bubbles which supply energy for ejecting ink droplets.
Another type of ink ejection is electrostatic ejection. An inkjet printer utilizing electrostatic ejection has energy units, each including two electrodes separated by a diaphragm and the underlying air layer. This type of inkjet printer applies a voltage across the two electrodes to deflect the diaphragm downward. The inkjet printer then turns off the voltage to release the diaphragm from the electrostatic force. As a result, the diaphragm returns to its original state with an elastic force which ejects ink droplets.
A further type of ink ejection is piezoelectric ejection. An inkjet printer utilizing piezoelectric ejection has energy units, each including a laminate of a diaphragm and a piezoelectric element having electrodes disposed on both surfaces thereof. This type of inkjet printer applies a voltage across the two electrodes so that the piezoelectric element produces a piezoelectric effect which induces a bending moment in the diaphragm. As a result, the diaphragm bends so as to eject ink droplets.
On the other hand, serial inkjet heads are known in view of the structure of inkjet heads. A serial inkjet head has hundreds of nozzles for each color. In recording, this type of inkjet head is moved perpendicularly to the direction in which printing paper is conveyed. The inkjet head, which is used alone, mechanically reciprocates (scans) substantially over the width of the printing paper to perform recording.
Line inkjet heads are also known. A line inkjet head includes many head units arranged along the width of printing paper. These head units are connected to form a single head with the length corresponding to the recording width. This type of inkjet head can achieve high recording speed because the head has a significantly larger number of nozzles than a serial inkjet head and does not involve mechanical scanning.
In particular, thermal line heads can achieve greatly higher recording speeds than thermal serial heads. Typical thermal inkjet heads repeat a temperature-increasing operation and a temperature-decreasing operation. The temperature-increasing operation is intended for instantaneously heating ink to a high temperature (about 330° C. to 350° C., which is the critical temperature for film boiling) to produce bubbles. The temperature-decreasing operation is intended for shrinking the bubbles produced by the film boiling to successfully separate ink droplets. These operations undesirably degrade the inkjet heads because the head temperature becomes excessively high after extended continuous recording.
Thermal serial heads therefore trade off recording speed for the control of temperature rise due to ink heating within a predetermined range. Thermal line heads, by contrast, allow high-speed, high-volume continuous recording because the resultant heat can be dispersed over the width of the heads, which are wider than serial heads.
General electronic devices have predetermined operating temperature ranges, temperature ranges in which the devices operate properly with performance according to their specifications; consumer electronic devices are generally guaranteed to operate properly at about 0° C. to 40° C.
Known inkjet printers, however, are generally guaranteed to operate properly in a relatively narrow temperature range, about 15° C. to 35° C. The lower limit of the operating temperature range is high because a water-based liquid ink freezes below the freezing point or, even if the ink does not freeze, exhibits high viscosity below 15° C.; water nearly doubles in viscosity or dynamic viscosity (hereinafter simply referred to as viscosity) as the temperature thereof decreases from 35° C. to 10° C. Below 15° C., the ink becomes difficult to eject in a droplet form, and thus the amount of ink ejected decreases.
On the other hand, the upper limit of the operating temperature range is low because the ink exhibits excessively low viscosity when the head temperature rises after, for example, extended continuous recording. When an ink prepared for use at 15° C. is heated to more than 35° C., the ink exhibits extremely low viscosity which increases the amount of ink ejected. This leads to a difference in print density between before and after extended recording.
This problem will be described in more detail. For a thermal inkjet head, the head temperature and the ink temperature generally exceed the ambient temperature because of self-heating during recording operation. These temperatures, however, are not necessarily as high during standby or immediately after power-up as those during the recording operation. Because the ink viscosity becomes high below a certain temperature, as described above, ejection conditions differ between the beginning and middle of recording. Accordingly, recording at lower temperatures often results in lower print densities while recording at higher temperatures often results in higher print densities.
This problem is more serious near the upper and lower limits of the recording range of printing paper. In low-temperature environments such as cold climates, particularly, the same amount of ink ejected as that at average temperature is difficult to ensure. In addition, the direction in which the ink is ejected can vary and, more seriously, the ink can cause ejection defects. In such cases, print defects such as white streaks and white spots appear in images printed on printing paper, thus degrading recording quality.
Electrostatic and piezoelectric inkjet heads, which utilize mechanical distortion, can supply ejection energy to ink irrespective of the ambient temperature, although the ambient temperature varies the viscosity of the ink. As a result, these types of inkjet heads exhibit poor ejection properties when suddenly driven from standby at low temperature. In that case, the use of an ink with high viscosity, which can move less easily, results in thin print areas or temporal ejection failure at the beginning of recording.
In addition to the problem described above, line heads have a problem associated with small head units connected in line. The production of a one-piece line head extending over the width of printing paper is not practical; a typical line head is composed of small head units arranged in line with the ends thereof connected. These head units share recording in the recording region of printing paper over the width. The sharing, however, leads to temperature variations between the individual head units, and density variations and white streaks become serious particularly for thermal line heads.
FIG. 9 is a diagram of an example of recording results obtained using a thermal line head of the known art in a low-temperature environment. As shown in FIG. 9, many white streaks appeared at the beginning of recording, and some of them are elongated in certain recording regions.
This problem results from the fact that the temperatures of more frequently used head units rise while those of less frequently used head units remain at the ambient temperature. That is, the head units of the thermal line head are usually used with different frequencies, and thus the ink temperature differs between the head units depending on the ejection frequencies thereof. Such temperature differences cause larger differences in ink viscosity and slight variations in the ejection properties and recording densities of the individual head units, leading to white streaks as shown in FIG. 9.
As described above, inkjet printers undesirably have narrow operating temperature ranges due to the increase in ink viscosity in a low-temperature environment. This problem has increasingly become serious with recent advances in the performance of inkjet printers. Recent inkjet printers have achieved higher recording densities with finer ink droplets. Accordingly, the number of ejection operations is increased to achieve higher recording densities without decreasing print density. For thermal line heads, in consequence, larger temperature differences occur between more frequently used head units and less frequently used head units, and thus white streaks appear more significantly.
On the other hand, the size reduction of nozzle holes for finer ink droplets leads to increased viscous drag of ink. In that case, the increase in ink viscosity in a low-temperature environment becomes more serious irrespective of the type of ink ejection (thermal ejection, electrostatic ejection, or piezoelectric ejection) or the structure of inkjet heads (serial heads or line heads).
It may be possible to arrange all head units for a thermal line head on a substrate with good thermal conductivity to reduce temperature differences between the individual head units. This approach, however, encounters another problem associated with thermal expansion. In general, materials with higher thermal conductivity tend to have higher thermal expansion coefficients. If a line head is constructed by bonding the base members of the head units, namely semiconductor substrates, to another substrate with a different thermal expansion coefficient, the head experiences significant thermal strain which can vary the ejection properties even within an operating temperature range of, for example, 15° C. to 35° C.
It may also be possible to perform preliminary ejection before recording to ensure predetermined ejection properties at the beginning of recording. The preliminary ejection, however, wastes a substantial amount of ink irrespective of recording on printing paper, thus increasing ink consumption and operating cost.
It may also be possible to supply preheat pulses (drive pulses with such a small width as to produce no bubbles) to heat-generating resistors of a thermal inkjet head to preheat the heat-generating resistors and thereby heat the ink to an appropriate temperature range before recording. This method, however, takes much time before recording (first print).
The technique of manually switching an inkjet printer between a high-quality recording mode and an immediate recording mode is known. If the temperature of an inkjet head is measured to be lower than a reference temperature in the high-quality recording mode, the inkjet head is preheated to the reference temperature or higher before recording. In the immediate recording mode, on the other hand, the inkjet head immediately starts recording.
According to Japanese Unexamined Patent Application Publication No. 2000-108328, for example, optimum recording can be performed for different applications by selecting a high-quality recording mode with sufficient preheating or a short-time rapid recording mode with some degradation in recording quality.
The technique according to the publication, however, has difficulty in simultaneously achieving improved recording quality and high-speed recording to constantly ensure high ejection stability. In addition, this technique undesirably complicates the overall system because special consideration is given to preheat the inkjet head only when the measured head temperature falls below the reference temperature. Furthermore, this publication makes no disclosure of the extension of the operating temperature ranges of inkjet printers.